Department of Chemistry

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The Latest Research Items

Phototherapeutics

Light-activatable drugs offer the promise of controlled release with exquisite temporal and spatial resolution. However, light-sensitive prodrugs are typically converted to their active forms using short-wavelength irradiation, which displays poor tissue penetrance. Researchers in the David Lawrence Group report in Angewandte Chemie, International Edition, on erythrocyte-mediated assembly of long-wavelength-sensitive phototherapeutics.

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The activating wavelength of the constructs is readily preassigned by using fluorophores with the desired excitation wavelength λex. Drug release from the erythrocyte carrier was confirmed by standard analytical tools and by the expected biological consequences of the liberated drugs in cell culture: methotrexate, binding to intracellular dihydrofolate reductase; colchicine, inhibition of microtubule polymerization; dexamethasone, induced nuclear migration of the glucocorticoid receptor.

 

More Fluorine - Better Solar Cell Efficiency

Developing novel materials and device architectures to further enhance the efficiency of polymer solar cells requires a fundamental understanding of the impact of chemical structures on photovoltaic properties. Given that device characteristics depend on many parameters, deriving structure/property relationships has been very challenging.

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Through an international collaboration, members of the You Group discovered that a single parameter, hole mobility, determines the fill factor of bulk heterojunction photovoltaic devices in a series of copolymers with varying amount of fluorine substitution. The continuous increase of hole mobility upon further fluorination is related to a preferential face-on orientation and improved pi-pi stacking of the polymer backbones. The results shows the potential of properly-designed polymers to enable high fill factors in thick devices, as required by mass production technologies. These significant results appeared in JACS, and were also featured in JACS Spotlights.

 

Quantum Dynamics on Supercomputers

In the perspective paper published in Computing in Science and Engineering’s special topic issue on Advances in Leadership Computing, researchers in the Kanai Group and his collaborators at University of Illinois at Urbana Champaign and Lawrence Livermore National Laboratory describe the state-of-the-art computational method for simulating quantum dynamics of electrons in complex materials using supercomputers.

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They discuss a new first-principles computational method for simulating quantum dynamics of electrons in complex materials by propagating time-dependent wavefunctions. The method is designed to take advantage of a large number of processing cores in today’s supercomputers by utilizing multiple levels of different parallelization schemes. They demonstrate a strong scaling of the computational method over 1 million processing cores on an IBM supercomputer. As an example of how new material properties can be investigated using this state-of-the-art method, non-equilibrium energy transfer rate from a fast proton to the electronic excitation in bulk gold was calculated and compared to available experimental data. Importantly, the computer simulation provides detail information on how the electronic excitation is induced by the fast proton. This new first-principles quantum dynamics method enables theoretical investigations into various non-equilibrium phenomena of electrons in large complex systems.

 

Selective Receptors

A new small molecule receptor, A2N, has been identified that binds specifically to trimethyllysine, Kme3, with sub-micromolar affinity. This receptor, as published in Organic & Biomolecular Chemistry was discovered by Nicholas Pinkin and Marcey Waters in the Waters Group, through the iterative redesign of a monomer known to incorporate through dynamic combinatorial chemistry, DCC, into a previously reported receptor for Kme3, A2B. In place of monomer B, the newly designed monomer N introduces an additional cation–Π interaction into the binding pocket, resulting in more favorable binding to Kme3 amounting to a ten-fold improvement in affinity and a five-fold improvement in selectivity over Kme2.

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This receptor exhibits the tightest affinity and greatest selectivity for Kme3-containing peptides reported to date. Comparative studies of A2B and A2N provide mechanistic insight into the driving force for both the higher affinity and higher selectivity of A2N, indicating that the binding of Kme3 to A2N is both enthalpically and entropically more favorable. This work demonstrates the ability of iterative redesign coupled with DCC to develop novel selective receptors with the necessary affinity and selectivity required for biological applications.

 

Optogenetic Engineering

Genetically encoded, light-activatable proteins provide the means to probe biochemical pathways at specific subcellular locations with exquisite temporal control. However, engineering these systems in order to provide a dramatic jump in localized activity, while retaining a low dark-state background remains a significant challenge.

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When placed within the framework of a genetically encodable, light-activatable heterodimerizer system, the actin-remodelling protein cofilin induces dramatic changes in the F-actin network and consequent cell motility upon illumination. In an article published in Angewandte Chemie, International Edition, researchers in the David Lawrence Group, demonstrate that the use of a partially impaired mutant of cofilin is critical for maintaining low background activity in the dark. They also show that light-directed recruitment of the reduced activity cofilin mutants to the cytoskeleton is sufficient to induce F-actin remodeling, formation of filopodia, and directed cell motility.

 

Temperature Matters for Bioceramics

To further improve the physical strength and biomedical applicability of bioceramics built on hydroxyapatite-gelatin, HAp-Gel, and siloxane sol-gel reactions, the You group incorporated mussel adhesive inspired polydopamine, PD, into the original composite based on HAp-Gel cross-linked with siloxane, discovered by Dr. Ching-Chang Ko at the UNC Dental School.

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Surprisingly, with the addition of PD, the team of You/Ko observed that the processing conditions and temperatures play important roles in the structure and performance of these materials. A systematic study to investigate this temperature dependence behavior discloses that the rate of crosslinking of silane during the sol–gel process is significantly influenced by the temperature, whereas the polymerization of the dopamine only shows minor temperature dependence. With this discovery, reported in the Journal of Materials Chemistry B, the team report an innovative thermal process for the design and application of these biocomposites..

 

Lubrication by Polyelectrolyte Brushes

Published in Macromolecules, Professor Michael Rubinstein, in collaboration with Ekaterina Zhulina with the Institute of Macromolecular Compounds, Russian Academy of Sciences in Saint Petersburg, describe the development of a scaling model relating the friction forces between two polyelectrolyte brushes sliding over each other to the separation between grafted surfaces, number of monomers and charges per chain, grafting density of chains, and solvent quality. They demonstrate that the lateral force between brushes increases upon compression, but to a lesser extent than the normal force.

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The shear stress at larger separations is due to solvent slip layer friction. The thickness of this slip layer sharply decreases at distances on the order of undeformed brush thickness. The corresponding effective viscosity of the layer sharply increases from the solvent viscosity to a much higher value, but this increase is smaller than the jump of the normal force resulting in the drop of the friction coefficient. At stronger compression the group members predict the second sharp increase of the shear stress corresponding to interpenetration of the chains from the opposite brushes. In this regime the velocity-dependent friction coefficient between two partially interpenetrating polyelectrolyte brushes does not depend on the distance between substrates because both normal and shear forces are reciprocally proportional to the plate separation. Although lateral forces between polyelectrolyte brushes are larger than between bare surfaces, the enhancement of normal forces between opposing polyelectrolyte brushes with respect to normal forces between bare charged surfaces is much stronger resulting in lower friction coefficient. The model quantitatively demonstrates how polyelectrolyte brushes provide more effective lubrication than bare charged surfaces or neutral brushes.

 

Layer-by-Layer Chromophore–Catalyst

As described in Chemical Science, members of the Dempsey Group, in collaboration with the Meyer Group, used a layer-by-layer procedure to prepare chromophore–catalyst assemblies consisting of phosphonate-derivatized porphyrin chromophores and a phosphonate-derivatized ruthenium water oxidation catalyst on the surfaces of tin oxide and titanium dioxide mesoporous, nanoparticle films. In the procedure, initial surface binding of the phosphonate-derivatized porphyrin is followed in sequence by reaction with a zirconium salt and then with the phosphonate-derivatized water oxidation catalyst.

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Fluorescence from both the free base and zinc porphyrin derivatives on tin oxide is quenched; substantial emission quenching of the zinc porphyrin occurs on titanium dioxide. Transient absorption difference spectra provide direct evidence for appearance of the porphyrin radical cation on tin oxide via excited-state electron injection. For the chromophore–catalyst assembly on tin oxide, transient absorption difference spectra demonstrate rapid intra-assembly electron transfer oxidation of the catalyst following excitation and injection by the porphyrin chromophore.